1. Field of the Invention
The present invention relates to a crystal oscillating circuit and an integrated circuit for crystal oscillation.
2. Description of the Related Art
FIG. 16 is an electric circuit diagram showing a conventional oscillating circuit using a crystal resonator. Numeral 161 designates a crystal resonator, numeral 162 designates a CMOS (Complementary Metal Oxide Semiconductor) invertor, numeral 163 designates a feedback resistor and numerals 164 and 165 designate capacitors. The crystal resonator 161 and the feedback resistor 163 are connected between input and output terminals of the CMOS invertor 162. Terminals on one side of the capacitors 164 and 165 are respectively connected to the input terminal and the output terminal of the CMOS invertor and terminals on the other side thereof are connected to a power source VSS (OV).
At present, according to such an oscillating circuit, downsizing of the crystal resonator has progressed in response to a need for speed-up of operational frequency and downsizing a total of a crystal oscillation module.
However, with downsizing a crystal resonator, a problem caused by crystal current (current flowing in a crystal in oscillation) cannot be neglected. For example, an increase in an amplitude by an excessive crystal current makes frequency unstable and brings about destruction of a crystal resonator in the worst case.
Further, although there has been proposed a circuit where the crystal current is reduced by providing a resistor 166 between the CMOS invertor 162 and the crystal resonator 161 as shown by FIG. 17, a resistance value thereof cannot be increased since the resistance value effects a significant influence on the negative resistance. Further, according to such a circuit, not only the crystal current but the amplitude voltage are lowered. Therefore, setting of the circuit becomes difficult in which the duty of a CMOS invertor at a next stage is significantly changed in view of a threshold voltage of the CMOS invertor at the next stage and the like.